?? T cells remain an enigma in the immune system. They often localize at epithelial barriers and are involved in the initial response to various infections. Indeed, a protective role of ?? T cells has been observed in various infectious model including our own studies with murine infection with Borrelia burgdorferi. ?? T cells also accumulate at sites of inflammation in autoimmune syndromes such as the synovium in rheumatoid and Lyme arthritis, the bowel in celiac disease, or the lung in sarcoidosis. Evidence suggests that ?? T cells are beneficial in autoimmune models. However, compared to ?? T cells, almost nothing is known regarding the specificity of ?? T cells or the structure of the TCR-??. We study Lyme arthritis in humans and mice as a model of the ?? T cell response during infection and potentially an autoimmune situation, as antibiotic-refractory Lyme arthritis closely resembles rheumatoid arthritis in synovial histology, HLA-DR4 association, and response to immunosuppressive agents. This project has a very focused mission to determine the structure and ligand(s) of the TCR-?? from two human Lyme arthritis synovial V?1 T cell clones using a newly developed soluble tetramerized TCR-??. We have produced initial crystals and also identified a ligand in host heat shock protein (Hsp) 70. Aim 1 will further characterize the interaction of the TCR-?? with its candidate ligand, Hsp70, using transfer of the TCR-?? to a Jurkat cell line, co-precipitation of TCR-?? with its ligand, reactivity to Hsp70 peptides, and plasmon resonance of TCR-?? with its ligand. In addition, we will use a second soluble TCR-?? to determine whether it recognizes the same ligand. Aim 2 will use the soluble TCR-?? to perform crystallization studies to determine the TCR-?? structure without and with its ligand, Hsp70. This will be done in collaboration with Dr. Roy Mariuzza, a specialist in the crystallizatio of TCR structures. This project uses novel reagents, being the only laboratory to our knowledge that has human synovial ?? T cell clones from Lyme arthritis patients, and a unique soluble TCR-?? from two of these clones. The proposed studies are very focused and feasible with a two-year period. In addition, we have extensive preliminary data to demonstrate the integrity of the soluble TCR-??, that it binds to activated monocytes, and we have produced crystals and identified a candidate ligand. Furthermore, all the required techniques are in place, as are the collaborators. The two aims are clearly related, yet independent of each other. The findings could provide an enormous advance for ?? T cell biology.